1. Technical Field
The present invention relates to peptides which may be used, for example, in the detection of free and complexed Prostate Specific Antigen (PSA) and thus in the diagnosis of prostate cancer.
2. Background Information
Prostate specific antigen (PSA) is a low molecular weight glycoprotein of approximately 30 kilodaltons (kD) produced mainly by prostatic glandular epithelial cells. PSA is a member of the human tissue kallikrein gene family, which consists of three genes, hKLK1, hKLK2, and hKLK3. The protein product of these genes are pancreatic-kidney kallikrein (hK1), human glandular kallikrein (hK2) (previously known as hGK-1), and PSA (hK3). PSA is a serine protease and exhibits a chymotrypsin-like specificity cleaving at a hydrophobic residue (see Chu et al., J. Urol. 141:152-56 (1989)). The other two kallikreins have trypsin-like specificity. These three genes are located on chromosome 19. The expression of both PSA and hK2 are under androgen control. The protein for hK2 has not been isolated, and its potential clinical utility has not been determined (McCormack et al., Urology 45:729-44 (1995); see also copending U.S. patent application Ser. No. 08/394,033).
PSA is produced in the prostate and occurs in high concentrations in prostate and seminal fluid. It also occurs in urine and in serum. The concentration of serum PSA in the normal male increases with age. Increased levels of serum PSA are found in benign prostatic hyperplasia (BPH), prostatis and prostate cancer. The expression of PSA in most females is negative or very low. While PSA isolated from seminal fluid or prostatic tissue occurs predominately as the 30 kD form of PSA, PSA in the serum occurs in various forms. A small portion of serum PSA occurs as low molecular weight or free PSA. The majority of immunodetectable PSA occurs as a 100 kD complex of PSA and alpha-1-antichymotrypsin (ACT) otherwise known as complexed PSA (PSA-ACT). Another PSA complex formed with alpha-2-macrogobulin is not detectable by current immunoassays. PSA complexes with these protease inhibitors through its serine protease active site. Presumably, the free PSA in serum is inactive PSA, since serum contains an excess of protease inhibitors (McCormack et al., Urology 45:729-44 (1995), Partin et al., J. Urol. 152:1358-68(1994), Lilja et al., Clin. Chem. 37:1618 (1991), Stenman et al., Cancer Res. 51:222 (1991) and WO 92/09136).
Serum PSA has become the most clinically useful tumor marker in prostatic disease. PSA is used to monitor the response to therapy and detect early relapses for prostate cancer. When prostate cancer is treated with radiation, surgery or androgen-deprivation, the serum levels of PSA decrease. The level that they decrease to following therapy is correlated to prognosis and survival. When used following radical prostatectomy, the PSA level following successful treatment should decrease to zero, since the prostate is the only significant source of PSA. This allows PSA to be a very sensitive marker for tumor relapse following surgery. PSA has also been used to help stage patients. It has been shown that PSA level correlates with tumor volume, but is not accurate enough to independently stage the disease; however, PSA level is useful in combination with other clinical and pathological parameters (Partin et al., J. Urol. 152:1358-68 (1994)).
A major development in the use of PSA is its application to the early detection of prostate cancer prior to the appearance of clinical symptoms. Prior to the advent of the use of PSA, the digital rectal exam was used (DRE). This method was not very sensitive or specific. In addition, many tumors detected by DRE were too large to cure. The use of PSA in conjunction with DRE as an early detection method has been validated in several clinical studies. Typically, a cutoff of 4.0 ng/mL has been used as the upper limit of normal. Above this level, about 33% of biopsies are positive, while above 10 ng/mL, approximately 67% are positive. PSA levels above a value of 4.0 can be due to prostatitis or benign prostate hypertropy (BPH). Most men over 50 years old have some evidence of BPH. Age-specific reference ranges for PSA have been suggested as a way to improve PSA's use in early detection (Parkin, et al., J. Urol. 152:1358-68 (1994)).
Patients with PSA below 4.0 ng/mL with a normal DRE are considered normal, while those above 10 ng/mL are considered likely to have prostate cancer. Therefore, the gray zone is between 4 and 10 ng/mL. Several methods have been devised to improve the specificity of PSA in early detection. They include: 1) the use of age-specific reference ranges, 2) the use of PSA changes over time, 3) the use of PSA density and 4) the use of ratios of PSA forms. It has been shown that the use of ratios of free PSA levels divided by total PSA levels or ratios of PSA-ACT complex levels divided by total PSA levels can improve the specificity of PSA for cancer. This is because the serum of patients with prostate cancer tend to have less free PSA than do the serum of men with BPH (McCormack et al., Urology 45:729-44 (1995), Partin et al., J. Urol. 152:1358-68 (1994), Lilja et al., Clin. Chem. 37:1618 (1991), Stenman et al., Cancer Res. 51:222 (1991) Christensson et al., J. Urol. 150:100-105 (1993) and WO 92/01936).
The peptides of the present invention will enable the production of antisera necessary to determine the amount of total PSA, free PSA and PSA-ACT complex present in a sample and thus improve the ability of the clinician to distinguish, for example, between BPH and prostrate cancer in a patient. The proper course of treatment may then be undertaken. This use and other uses of the present peptides will be described in further detail below.
All U.S. patents and publications referred to herein are hereby incorporated in their entirety by reference.